Concrete bridge deckings are commonly coated with a protective layer of asphalt (up to 6 cm thick), uniformly spread under heavy rolling equipment, which may weigh 10 metric tons or more. This asphalt layer is exposed to extremes of weather throughout the year, and will eventually develop cracks, some so small as to be practically invisible, others much larger. All are harmful, in that they permit liquid water to penetrate down to the concrete surface, where it freezes in winter and causes sizable portions of the asphalt layer to spall away. Alkali from the concrete accelerates asphalt deterioration. These adverse factors (freeze-shattering, traffic impact, chloride disintegration from de-icing salt, etc.) result in serious damage to the concrete traffic surface and eventually require major repairs.
Aside from asphalt-coated concrete surfaces, bridge concrete in the form of superstructure, parapets, crash barriers, etc., is also subject to attack, viz., from air- or moisture-borne industrial chemicals; spattered de-icing salt; and carbonation, i.e., gradual penetration of atmospheric carbon dioxide which then reacts with the alkaline materials in the concrete and attacks reinforcement in the concrete.
To inhibit the aforesaid destruction, it is conventional in concrete bridge construction and maintenance to apply a bridge deck membrane (BDM) to the concrete surface before laying down asphalt. Several BDM's are available. Polyvinylchloride is a typical conventional commercial coating. Polyurethane has been tried. A BDM should meet a number of technical and economic criteria. It should:
(1) be impermeable to liquid water from above, yet be sufficiently permeable to permit small amounts of water vapor to escape from the concrete substrate; PA1 (2) be solventless; PA1 (3) be easily applied, preferably sprayable; PA1 (4) have good adhesion to concrete; PA1 (5) have low chloride penetration; PA1 (6) be stable to concrete alkali; PA1 (7) be stable under conditions of asphalt application - hard, but not brittle, yet be sufficiently flexible to cope with dimensional changes generated by temperature differentials and bridge movements; be able to tolerate application of asphalt at 170.degree. under a 10-ton roll; PA1 (8) be resistant to asphalt migration (i.e., tendency of low molecular weight hydrocarbons in asphalt to migrate into the BDM, weakening and/or destroying it). PA1 (9) have low raw material costs; PA1 (10) not discolor concrete; PA1 (11) have prolonged life on exposed surfaces (e.g., vertical surfaces and other surfaces not asphalt-coated); PA1 (12) be tough enough within a reasonable time for construction crews to walk on it.
In a brochure entitled "Bayer Engineering Polymers" (apparently dated November, 1985), Bayer UK Limited offers commercially a polyurethane made from a polyether polyol (Component A) and a modified MDI (Component B). The composition is not further given. The two components are mixed at the site, e.g., by spraying, on bridge decking or other concrete surface. The applied resin is said to cure tack-free in a few minutes and can be walked on in 20-30 minutes.
In a technical brochure on "352-Oldopren-S" (apparently dated March, 1983), Buesing & Fasch GmbH & Co. of Oldenburg, Germany, describe an MDI-based 2-component, polyurethane that provides an elastic film, useful (under asphalt) on road- and bridge-concrete surfaces.
R. I. Frascoia describes the use of four polyurethane membranes in bridge deck systems in an article, "Field Performance of Experimental Bridge Deck Membrane Systems in Vermont", Transportation Research Record, pp. 57-65 (1984). Three of the polyurethanes were asphalt-modified. Formulations are not otherwise given. Bond between bituminous pavement and membrane was rated "Poor", but overall performance was rated "Fair to Good".
Use of an "asphalt-extended urethane membrane" is described in an article by A. L. Meader, Jr. et al, "Development of a Cold-Poured Bridge Deck Membrane System", ASTM Special Technical Publications N 629, pp. 164-177 (1976).
For a good review article, especially for UK practice, see M. D. McDonald, "Concrete Bridge Deck Waterproofing Systems: in Highways and Road Construction," pp. 26-30 (August 1973). According to the article, polyurethane is blended with pitch to improve low-temperature flexibility and to reduce raw material costs; the membrane may need an epoxy primer (on concrete) and may need a surface protective layer before rolling on the final asphalt coating. "Cracking" and "chisel" tests are described.
In a technical bulletin, "Hycar Reactive Liquid Polymers" released by BFGoodrich Co. (apparently dated March, 1981), Hycar polymer is described as an acrylonitrile-based diol that can be reacted with MDI to provide low temperature flexibility and chemical resistance.
U.S. Pat. No. 4,608,203 (1987) discloses a polyurethane coating for bridge concrete, prepared from polyols and MDI. The polyols can be a mixture of poly(propylene oxide) (col. 1, line 65), glycerine (col. 2, line 3), and acrylonitrile-butadiene copolymer (col. 2, line 35).
U.S. Pat. No. 4,689,268 (1987) discloses a 3-layer laminate on concrete, viz., epoxy resin plus filler, a bonding agent, and a polyurethane.
U.S. Pat. No. 4,559,239 (1985) describes a 2-component (polyol-MDI) polyurethane applicable to concrete.
U.S. Pat. No. 3,725,355 discloses glycerine, polyether polyol, and an isocyanate prepolymer. The polyol can be a triol and must have a molecular weight of at least 2,000.
U.S. Pat. No. 4,507,336 (1985) describes a 2-component ("A" and "B") polyurethane, sprayable as a roof coating. "A" is (e.g.) a liquid modified MDI plus a chlorinated paraffin; "B" comprises (e.g.) a polypropylene glycol, 1,4-butanediol, and dibutyltin dilaurate catalyst.
Canadian Pat. No. 927,642 (1973) describes applying a polyurethane foam directly to a roadbed (not to concrete), followed by a bitumen layer on the polyurethane foam.
The combination of MDI and poly(propylene oxide) triol is disclosed, e.g., in U.S. Pat. Nos. 3,515,699, 4,532,316, and 4,604,3445. The combination of glycerol and poly(propylene oxide) triol is disclosed in U.S. Pat. Nos. 3,993,576 and 4,410,597, and the combination of MDI and glycerol in 4,145,515. Glycerol, poly(propylene oxide) triol, and MDI are disclosed in 4,225,696, 4,376,834, 4,436,896, and 4,551,498. In each of the above references our individual reactants appear in lists that are generally long and comprehensive (i.e., in so-called "shot-gun" disclosures). Aside from reactant ratios (crucial in our invention), the permutative possibilities of the reference lists run into the hundreds of thousands, perhaps millions, with no suggestion as to how to select our special combination from the myriad possibilities.
Description of Certain of our Components
PAPI-94 is an MDI, viz., a mixture of 2 isomers, 98% of the 4,4-isomer of diphenylmethane diisocyanate and 2% of the 2,4-isomer; available from Dow Chemical Co. Other mixtures of these same two isomers can be used, preferably with the 4,4'-isomer as the major component, e.g., 95/5 (meaning 95 wt. % of the 4,4'-isomer and 5% of the 4,2'-isomer); 85/15, 80/20, etc.
Mondur XP-744 is a modified MDI. It can be made by reacting together diphenylmethane 4,4'-diisocyanate (MDI) with a poly(oxypropylene) glycol of the formula: ##STR1## The value of n lies between 0 and 3, approximately indicating that the material is a mixture, probably with small amounts of higher and lower molecular weight polymers of the same genus. In the main reaction the MDI end-caps both ends of the poly(oxypropylene) glycol. The prepolymer thus formed may also contain small amounts of other products of the reaction, containing isocyanate and/or hydroxyl groups. The prepolymer is typically K--O--(CH.sub.2 CH[CH.sub.3 ]O).sub.m --K where K is ##STR2## and m is about 2-5. In general, this material may be described as an MDI/polyisocyanate material comprising about 45-50 weight % MDI with the balance being the described prepolymer, i.e., with some prepolymer molecules being relatively small and others larger (depending generally on the amount of capped polyoxypropylene), but with the averages being substantially as above enumerated.
Whereas MDI is a solid, and is very difficult to handle in spray apparatus, the above described MDI/polyisocyanate prepolymer is a homogeneous liquid and is readily sprayed. It is available as Mondur XP-744 from Mobay Corporation.
Hycar 1300X17 is a long chain acrylonitrile-butadiene hydroxyl-terminated diol (copolymer with 17% acrylonitrile) available from BFGoodrich Co., Cleveland, OH. Weight average molecular weight is about 4,000, viscosity, 140,000 cp; hydroxyl number, 25 mg KOH/g; residual carboxyl acid number, 3.9.
NIAX LG-168-- or simply LG-168, is a poly(oxypropylene) triol. It contains only secondary hydroxyl groups and has an average molecular weight of about 1,000; available from Union Carbide Corp.